This invention relates to pneumatic tires and in particular but not exclusively to tires for automobiles.
It is known that tires operate in conditions in which the inside and outside of the tire are subject to different operating conditions, for example during cornering. The terms inside and outside of a tire refer to the positions as on a vehicle e.g. the outside of the tire is that side of the tire facing outwardly of the vehicle.
It is known therefore to have different tread patterns on the inside and outside portions of the tire tread in order to compensate for these differences, for example, in U.S. Pat. No. 3 155 135 there is shown a tire with a tread portion having circumferential ribs on the outer side portion of the tread pattern and a block pattern on the inside portion of the tread pattern.
It is also known to have tread patterns formed by the repetition of an elementary shape to give an asymmetrical total tread pattern.
A disadvantage of some asymmetrical tread patterns is that the ratio of the groove area to the area of the interposed surfaces differs between the inside and outside portions of the tire.
In accordance with the invention there is provided a pneumatic tire comprising a tread having a pair of lateral edges, and a mid-circumferential plane, said mid-circumferential plane dividing the tread so that one side of said plane is the inside portion of the tread and the other side of said plane is the outside portion of the tread, said tread having a plurality of circumferential grooves therein spaced axially apart across the tread and each of which has one of two differing configurations, a groove having the first of said two configurations being located on one side of the plane and having a greater width than a second groove of the first configuration, and a groove having the second of said two configurations and being located on the other side of the plane and having a greater width than a second groove of the second configuration, wherein the ratio of groove area to tread area on the inside and outside portions of the tread is substantially equal.
By having one wider groove located on each side of the plane it is possible to equalize the total areas of grooves on each side of the mid-circumferential plane of the tire.
Preferably the second groove of the first configuration is located on said other side of the plane, and the second groove of the second configuration is located on said one side of the plane.
Preferably the circumferential grooves are in the forms of straight grooves and zig-zag grooves. In a tire having an asymmetrical tread the mid-circumferential plane divides the tire so that the tread on one side of the plane can be referred to as an inside portion of the tread, and the other side of the plane as an outside portion of the tire, and preferably the wider straight groove is located in the inside portion of the tread and the wider zig-zag groove is located in the outside portion of the tread.